Hepatitis C virus (HCV) is responsible for a large proportion of the chronic liver disease worldwide and accounts for 70% of cases of chronic hepatitis in industrialized countries. The global proportion of hepatitis C is estimated to average 3% (ranging from 0.1% to 5.0%); there are an estimated 170 million chronic carriers throughout the world. There is a continuing need for effective therapeutic agents against HCV. Standard therapy for hepatitis C infection presently consists of combination therapy with an antiviral, ribavirin, and an immunomodulatory interferon derivative.
WO 01/45509 (J. Lau et al.) discloses L-nucleosides with in vivo antiviral activity against HCV. Levovirin (1-(3S,4R-dihydroxy-5S-hydroxymethyl-tetrahydro-furan-2S-yl)-1H-[1,2,4]triazole-3-carboxylic acid amide; Ia), is the L-isomer of the antiviral nucleoside ribavirin (II). Unlike ribavirin, levovirin does not have direct detectable antiviral activity; however, levovirin stimulates immune responses by enhancing antiviral Th1 cytokine expression. Levovirin appears to lack toxicity associated with ribavirin.

While nucleoside derivatives frequently possess high levels of biological activity, their clinical utility is often hampered by suboptimal physical properties and limited bioavailability requiring large doses at frequent intervals to maintain therapeutically effective levels. Chemical modification of the nucleoside can alter the physicochemical properties of the compound and improve the efficiency and selectivity of drug delivery.
Esters of neutral amino acid acids have been found to actively transported across the intestinal mucosa by amino acid transporters. Colla et al. (J. Med. Chem. 1983 26:602-04) disclose the preparation of water soluble ester derivatives of acyclovir. L. M. Beauchamp et al. (Antiviral Chem. & Chemother. 1992 3(3):157-64) disclose eighteen amino acid esters of the antiherpetic drug acyclovir and identified the L-valyl ester of acyclovir as the best prodrug among the esters investigated.
EP 0 375 329 (L. M. Beauchamp) disclosed the preparation of the bis-iso-leucine ester of gangciclovir. U.S. Pat. No. 6,083,953 (J. J. Nestor et al.) discloses a crystalline from of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-L-valinate hydrochloride
WO 00/23454 (A. K. Ganguly et al.) disclose bioreversible prodrugs of ribavirin II. Compounds in which the 5-hydroxy of II is esterified to natural and unnatural amino acids are disclosed. U.S. Pat. No. 6,423,695 (R. Tam et al.) disclose methods of treating a patient with a virus infection by administering amidine prodrugs of ribavirin.
WO 01/68034 (G. Wang et al.) disclose bioreversible phosphorylated and non-phosphorylated prodrugs of levovirin. 5-Acyl and 2,3,5-triacyl compounds are disclosed and 5-amino acid esters are also described generically. U.S. Ser. No. 60/432,108 discloses acylated prodrugs of levovirin.
Salts of acidic and basic compounds can alter or improve the physical properties of a parent compound. These salt forming agents, however, must be identified empirically by the pharmaceutical chemist since there is no reliable method to predict the influence of a salt species on the behavior of a parent compound in dosage forms. Effective screening techniques, which potentially could simplify the selection process, are unfortunately absent (G. W. Radebaugh and L. J. Ravin Preformulation. In, Remington: The Science and Practice of Pharmacy; A. R. Gennaro Ed.; Mack Publishing Co. Easton, Pa., 1995; pp 1456-1457).
Polymorphism is the ability of any element or compound to crystallize as more than one distinct crystalline species. Different polymorphic forms of salts are frequently encountered among pharmaceutically useful compounds. Physical properties including solubility, melting point, density, hardness, crystalline shape and stability can be quite different for different polymorphic forms of the same chemical compound.
Polymorphic forms are characterized by scattering techniques, e.g., x-ray diffraction powder pattern, by spectroscopic methods, e.g., infa-red, 13C nuclear magnetic resonance spectroscopy and by thermal techniques, e.g, differential scanning calorimetry or differential thermal analysis. The compound of this invention is best characterized by the X-ray powder diffraction pattern determined in accordance with procedures which are known in the art. For a discussion of these techniques see J. Haleblian, J. Pharm. Sci. 1975 64:1269-1288, and J. Haleblain and W. McCrone, J. Pharm. Sci. 1969 58:911-929. Although the intensities of peaks in the x-ray powder diffraction patterns of different batches of the valinate ester Ia may vary slightly, the peaks and the peak locations are characteristic for a specific polymorphic form.
The problem which must be solved is to identify a suitable salt and/or polymorph which (i) possesses adequate chemical stability during the manufacturing process, (ii) is efficiently prepared, purified and recovered, (ii) provides acceptable solubility in pharmaceutically acceptable solvents, (iii) is amenable to manipulation (e.g. flowability and particle size) and formulation with negligible decomposition or change of the physical and chemical characteristics of the compound, (iv) exhibits acceptable chemical stability in the formulation. In addition, salts which contribute minimally to the molar weight so that the resulting material comprises a high molar percent of the active ingredient are highly desirable since the quantity of material which must be formulated and administered to produce a therapeutically effective dose is minimized. These oft conflicting requirements make identification suitable salts a challenging and important problem which must be solved by the skilled pharmaceutical scientist before drug development can proceed in earnest.